The Three Kings

The Three Kings are Upper S-Class mazoku demons on equal footing (although Raizen in his prime would have been easily the strongest of the three) with vast armies of loyal subjects of A-Class soldiers and some S-Class demon generals, who each have a strong connection to one of the three members of Team Urameshi with a demonic heritage. Each King suffered a physical loss to obtain their enlightenment; some are more pessimistic than others, but all have gained enough patience to avoid fighting one another, knowing that if one fought another to the death, the third would strike at the moment of weakness and be the remaining King. This situation lead to a 500 year stalemate that lasted until Raizen's death, upon which his descendant Yusuke called a tournament that caused them to disband their vast kingdoms to set up a tournament for the absolute ruler of the Makai.

At the finale, none of the three are surpassed by the heroes of the series and remain the strongest characters in the series, especially the deceased Raizen, whose descendant, even with the immense power he gains at the finale, is still inferior to his ancestral father (Mukuro had weakened considerably due to a change of heart, but is still far superior to the majority of the characters and Yomi held back his strength simply to enjoy the heat of a good battle).

The only possible contenders to rival the power of the Three Kings in the series are the old friends of Raizen. At the time Raizen gave up fighting, his comrades lost their passion for it as well and all went into seclusion and broke ties with him. However, centuries later at his funeral, it is revealed that despite their lack of practice, each of Raizen's friends were able to at least match Yomi in power, implying that in the past, they exceeded the powers of even Yomi and Mukuro.

Raizen is either the strongest or 2nd strongest of the Three Kings (as well as the strongest or 2nd strongest character in the series, challenged only by a raged Mukuro), and Yusuke's ancestor. Hundreds of years before the series, he is injured and later nursed back to health by a human woman. He falls in love and has a child with her, vowing to never eat humans again. He starves and weakens over hundreds of years. Yusuke eventually appears and Raizen trains him until his death. His kingdom is passed on to Yusuke, who manages to avoid war with the other kings. He is voiced by Takayuki Sugō in Japanese media and Chris Sabat in English media. His base Youki level is 1,322,000 in a weakened state, though worthy to note is that much of his shortcomings are in the field of Special Abilities, while his Stamina and Offensive Power remain the highest despite his hunger. His right hand man is Hokushin.

Mukuro is the female member of the Three Kings. After being abandoned at birth, she is adopted by another demon and turned into a sex slave. She eventually pours acid on her body to escape his desires, which leads to him placing extremely strong shackles on her. She escapes, eventually replaces the right side of her body with metal and machinery, assumes the identity of a man, and becomes a ruthless killer. She eventually finds Hiei while preparing for war, placing him as her second in command, and becomes close to him. During the tournament, she faces Hiei and wins, though his main goal is to break her shackles. This quells much of her hatred, which leads to her eventual loss--her power is directly proportional to her rage, thus making her weaker as her rage lessens. Afterward, Hiei captures her adoptive father, whom she can not kill because of special mental modification on her mind, and merges him with a plant that constantly heals him, allowing her to torture him at her pleasure. She is voiced by Minami Takayama in Japanese media and Wendy Powell in English media. Her base Youki level is 1,575,000. Her former right hand man was Kirin but was replaced by Hiei.

Yomi is the third of the Three Kings. He was once partners and a follower to Yoko Kurama, however he is blinded by an assassin Kurama hired to kill him. Despite this, he grows in power to become equal with Mukuro and Raizen. His dream is to unite the three worlds under a single ruler, mainly himself. Because of this, he recruits Kurama as his second-in-command. Prior to the Makai tournament, he creates a child, Shura to increase his chances of winning, but instead faces him and emerges victorious. In the finals of the tournament, he faces Yusuke and defeats him after fighting for sixty hours. However, the battle left him so exhausted that he lost the successive battle to Koukou. His base Youki level is 1,550,000. His right hand man was Shachi and is later replaced by Kurama.